1. Field of the Invention
The present invention relates to a communication system. More specifically, the invention relates to a communication system for transmitting and receiving burst signals between a round-trip (or a lower orbit) satellite and a communication station, such as a ground station and so forth.
2. Description of the Related Art
A communication system in point-to-multi-point connection for communicating through a communication satellite typically uses a stationary satellite on a geostationary orbit. There has also been proposed a system to perform communication through a round-trip satellite group.
In the recent years, it has become possible to launch small satillites economically for establishment of technology for launching small size satellites together with a large satellite associated with enlargment of the vehicle for the satellite, and also for development of a vehicle for launching the small size satellite. Accordingly, a satellite communication system employing low height level or middle height level round-trip satellite has been attracting attention.
Such a satellite communication system is advantageous in comparison with the system employing the stationary satellite with respect to lower transmission delay and down-sizing of a communication terminal. For instance, Iridium project of Motrola Corporation (U. S. A.) as introduced on "Nikkei Communication," Nikkei BP K. K. Oct. 21, 1991, No. 12, pages 31 to 32 is one example of the system employing the round-trip satellite group.
FIG. 5 shows a network construction of a typical satellite communication system employing low height level (lower orbit) round-trip satellite group. Here, 101 to 103 denote round-trip satellites, 104 to 107 denote network control stations present at least one in each area capable of communication to each of the round-trip satellites 101 to 103. The network control stations 104 to 107 serve as a gate station for connecting communication channels of the satellite to a subscriber network, charge management and management of network subscribers.
Among these, the control station 107 is a network base station managing a whole network. 108 denotes a compact communication terminal which can be hand carried or installed on a transportation means by a subscriber, and 109 denotes the subscriber network. H denotes the earth.
Each of satellites 101 to 103 are satellites moving in the direction Y with irradiating spot beams. On the terrestrial surface, cells A1 to G1, A2 to G2, A3 to G3 are formed by respective spot beams. This is why the satellite communication employing the round-trip satellite group is called satellite cellular. As is well known in terrestrial cellular, each cellular repeatedly uses several mutually different frequencies for effective use of a frequency band. Also, by using the spot beam, power of the satellite can be efficiently used. On the drawings, there is illustrated a condition where communication is performed between the communication terminal 108 and the control station 105 as shown by arrows Y1 to Y3.
Each distance between center points to the satellite is differentiated per cell. Therefore, propagation time between the communication terminal and the satellite is differentiated per cell.
FIG. 6 shows a condition of the round-trip satellite group network after expiration a certain period from the condition of FIG. 5. Namely, due to elapse of time, respective satellites have moved. Then, the communication terminal 108 located at a position within a cell A2 of the satellite 102 becomes to be placed within the cell C1 of the satellite 101. To accommodate the change, communication as shown by arrows Y1 to Y4 is performed.
FIG. 7 shows a condition of the round-trip satellite group network after further expiration of the certain period from the condition of FIG. 6. In this condition, while the satellites have moved, the satellite to be used is held unchanged. Namely, the communication terminal 108 which initially placed within the cell C1 of the satellite 101, is now placed at the position within the cell B1 of the same satellite.
Thus, in the satellite communication system employing a low height level round-trip satellite group, when continuous communication is to be performed, since the satellite moves relative to the communication terminal, a distance between the satellite and the communication terminal under communication varies from time to time. Therefore, signal propagation delay is also varied correspondingly.
On the other hand, when the communication terminal moves, the cell in which the communication terminal is present, may change according to movement. However, this condition may also be considered that the satellite has moved relative to the communication terminal.
Here, in the satellite communication system employing the low height level round-trip satellite group, a system for dividing channel on a time axis while occupying a common frequency channel is employed. Examples include, a time-divided multi-access (TDMA) communication system or slotted aloha communication system and so forth. In these systems, a plurality of ground stations (communication terminals) distributed in a geographically wide area involved in the communication system have to transmit and receive burst signals. Accordingly, it is required that the burst signals transmitted from a plurality of ground stations are aligned on time slots in a non-overlapping manner on a time axis on the communication satellite.
In order to align the burst signals from a plurality of ground stations on a plurality of time slots in non-overlapping manner on the time axis on the satellite, it becomes necessary to accommodate all of differences of propagation delay of the ground stations spread in wide area. For this purpose, it may be considered to set a guard time as long as possible in a unit time slot. However, this clearly degrades data transmission efficiency. Therefore, it becomes necessary to optimally adjust a "time shift" of a transmission timing to transmit the burst signal depending upon the magnitude of the propagation delay.
This will be discussed with reference to the drawings.
FIGS. 8 and 9 are explanatory illustrations for aligning the burst signals on a plurality of time slots on the time axis on the communication satellite.
FIG. 8 shows the case where ground stations located at different places transmit the burst signals (hatched portions) according to a common reference transmission timing C. In this case, due to difference of the propagation periods to the communication satellite S from the grounding stations A and B, the burst signals are aligned on a plurality of time slots in partially overlapped condition on the time axis of the satellite S.
FIG. 9 shows the case where the ground stations located at different places transmit the burst signals (hatched portions) by adding own specific offset magnitudes TA and TB for the reference transmission timing C. In this case, even when the propagation delays from the ground stations A and B to the satellite are different, the burst signals may be aligned on a plurality of time slots without causing overlap on the time axis of the satellite S. It should be noted that TGT is a guard time between the burst signals.
Conventionally, the above-mentioned offset magnitude is linearly determined as a fixed values depending upon the positions of the ground stations, as set out in Japanese Unexamined Patent Publication (Kokai) No. Heisei 1-181336, left upper column, line 18 to right upper column, line 10.
When it is desired to continuously perform communication in the communication system employing low height level round-trip satellite group, since the satellites sequentially move relative to the communication terminal, the distance between the satellite and the communication terminal is varied according to elapsing of the time period. Thus, the propagation delay of the signal is varied.
In order to accommodate variation of the propagation delay, it has been considered that the guard time TGT between the burst signals be increased. However, this inherently causes significant degradation of the transmission efficiency. Therefore, as set forth above, even when the burst signal is transmitted with the addition of a specific offset in a common reference timing, if the offset magnitude TA and TB in FIG. 9 are the fixed values, it cannot be adopted to vary with propagation delay according to elapsing of the time. Therefore, burst signals may cause mutual collision on the time axis of the communication satellite, to make it impossible to align the burst signals on a plurality of time slots.